Trimethylaluminum (TMA) is increasingly attracting attention because of its many applications in the areas of semiconductors, aluminizing by electrodeposition, and catalysis in polyolefin production.
A great many methods of preparing trimethylaluminum are described in the literature, for example, in U.S. Pat. Nos. 2,744,127 and 2,839,556; Adv. Inorg. Chem. Radiochem. 7, 269 (1967); and Zeiss, Organomet. Chem., ACS Monograph No. 147, 197 (1960).
On the industrial scale, trimethylaluminum is usually produced by reduction of methylaluminum chlorides, and primarily dimethylaluminum chloride (DMAC), with molten metallic sodium, dispersed in an inert hydrocarbon such as Tetralin.RTM. (tetrahydronaphthalene), n-decane, n-heptane, etc., according to the reaction equation EQU 3Me.sub.3 -.sub.x AlCl.sub.x +3.times.Na.fwdarw.(3-.times.)Me.sub.3 Al+.sub.x Al+3.times.NaCl. (1)
The methylaluminum chlorides to be used include dimethylaluminum chloride (DMAC) and methylaluminum sesquichloride.
Because of passivation effects due to the deposition of reaction byproducts (NaCl Al) on the surface of the sodium, however, the yields are moderate (47 to 85 percent), and the trimethylaluminum obtained is contaminated by residues of the starting product dimethylaluminum chloride, and of the inert hydrocarbon which is often used in large amounts as a suspending agent. Besides, antipollution and industrial safety considerations make it advisable to dispense with readily flammable and toxicologically questionable solvents.
Using sodium in excess promotes the formation of complex alkyls, which reduces the yield still more: EQU 4Me.sub.3 Al+3Na.fwdarw.3NaAlMe.sub.4 +Al (2)
In addition, reactor fouling as well as waste disposal and safety problems due to included sodium are observed. U.S. Pat. No. 5,015,750 relates to a process by which trimethylaluminum is obtained from methylaluminum chlorides and sodium in yields of about 90 percent without using sodium in excess. The product contains only about 0.03 percent chlorine and the synthesis requires considerably less suspending agent than comparable processes. However, such favorable results are obtained only in the presence of from 1 to 10 percent of catalysts such as alkali-metal and alkaline-earth fluorides, which must also be taken into consideration in connection with waste disposal.
Moreover, in the operating procedure there proposed, dimethylaluminum chloride and sodium are fed in simultaneously, the sodium being directed through the gas space onto the liquid surface. Now if the sodium is distributed by an agitator, for example, over the reactor wall above the liquid surface, the sodium will immediately react with the dimethylaluminum chloride, present in the gaseous phase. With this process repeating itself continually, considerable buildup will occur.